This statement is not true. Although most of the countries in the world have become democratic after the Second World War, many international institutions are still not working on democratic principles.
In the United Nations Organisation (UNO), enjoyment of veto power by five permanent members (US, Russia, UK, France and China) of the Security Council is undemocratic. Some other agencies like International Monetary Fund (IMF), World Bank and Word Trade Organisation (WTO) also are not working on democratic principles.
The voting power of the IMF and World Bank depends upon how much money each country has contributed to these organisations.
After the collapse of the Union of Soviet Socialist Republic, USA is the only super power left in the world. This has enabled and encouraged that country to act unilaterally without seeking the consent of other countries.